Polymer compositions for use in tire treads have an exacting set of technical specifications to meet. The tires are required to have a high degree of traction with respect to both wet and dry surfaces (skid resistance), and to retain their rubbery characteristics over a wide range of temperatures. They must be tough and wear resistant, to stand up to the shock and abrasive forces commonly encountered in service. They should have low hysteresis characteristics, i.e. they should have good rebound characteristics and resilience, to provide low rolling resistance and heat build-up, thereby reducing fuel consumption. Some of these requirements are essentially incompatible with one another, for example, reduced rolling resistance and high wet skid resistance.
At all times, the rubber compositions must be readily processable, to allow for their mixing with regular compounding ingredients, such as carbon black and curing systems, and be sulfur-curable, so as to be economically prepared and manufactured. As the requirements for automobile safety and low fuel consumption become more demanding, the specifications for tire tread rubber compounds become more exacting.
Natural rubber and/or sulfur-curable synthetic rubbery polymers, based upon conjugated diolefinic monomers, are commonly used as the elastomeric polymers in automobile tire tread compositions. Each individual rubbery polymer provides vulcanizates which are deficient in one or more of the physical properties being sought. For example, high cis-1,4-content polybutadiene provides vulcanizates of good wear resistance and good resilience, along with an acceptably low glass transition temperature, so that the vulcanizate retains its rubber properties down to acceptably low temperatures. Such vulcanizates are, however, lacking in both wet and dry traction, except perhaps at very low temperatures, e.g. on ice. High vinyl content polybutadienes, i.e. those having about 70 molar percent of 1,2-content, on the other hand, are deficient in wear resistance and resilience. In addition, the glass transition temperature thereof is too high--i.e. polybutadiene having about 70 molar percent of 1,2-content has a glass transition temperature of about -33.degree. C., which does not permit its use in colder climates.